Phosphate glass compositions are well known to those skilled in the art. Thus, for example, U.S. Pat. No. 4,645,749 of Drake discloses a water soluble phosphate glass composition useful for preparing an analytical solution containing phosphorous pentoxide, calcium oxide, and sodium oxide. When this composition is contacted with pure water, it releases the sodium ion at a rate of from 0.01 to 100 micrograms per square centimeter per hour.
Fibers made from phosphate glass compositions often have poor physical properties. Thus, for example, U.S. Pat. No. 4,604,097 of Graves et al. disclose a spun or drawn fiber consisting primarily of calcium oxide and phosphorous pentoxide which may be used as reinforcement for resorbable polymeric bone plates and artificial ligaments. In column 5 of the patent, the patentees disclose that the average tensile strength of the fibers prepared in the experiment of Example I was only 37,100 pounds per square inch. In column 6 of the patent, it is disclosed that the average tensile strength of the fibers prepared in the experiment of Example 11 was only 57,800 pounds per square inch.
U.S. Pat. No. 4,613,577 of Hideo Tagai et al. discloses a bioabsorbable glass fiber containing calcium phosphate; similar glass fibers are disclosed in Tagai et al.'s U.S. Pat. Nos. 4,735,857 and 4,820,573. Tagai et al. do not disclose the physical properties of their glass fibers and how durable such fibers are. However, there is much evidence in the Tagai et al. patents that at least some of the calcium phosphate glass fibers have poor mechanical properties. Thus, at lines 11-15 of column 4 of U.S. Pat. No. 4,613,577, Tagai et al. disclose that "Even if fibers might be formed from a glass having a molar ratio of Ca/P of not less than 0.6, the fibers become opaque due to devitrification and too weak to be applied for practical use." At columns 4 and 5 of such patent, Tagai et al. teach that it is preferred to coat the fibers of their invention with calcium phosphate. However, at column 5, Tagai also teaches that the pH of the coating solution is critical because, if " . . . the pH value of the solution is less than 2, the fiber glass mainly composed of calcium phosphate deteriorates so as to have a strength weaker than that required for practical use."
Furthermore, many of the compositions discussed in the Tagai et al. patents cannot be formed into fiber. Thus, e.g., on lines 1-8 of column 4 of the U.S. Pat. No. 4,613,577, Tagai et al. teach that "The fiber glass . . . should have a molar ratio of Ca/P of not less than 0.2 and less than 0.6 . . . If the molar ratio of Ca/P is less than 0.2, the viscosity of the molten glass becomes so low as to make it difficult to form fibers therefrom. On the contrary, if the molar ratio of Ca/P is not less than 0.6, it becomes hard to melt the glass or the viscosity of the molten mass becomes too high to spin fibers therefrom.
It is an object of this invention to provide fiberizable phosphate glass compositions with superior mechanical properties and chemical durability.
It is another object of this invention to provide bioabsorbable glass fibers with superior mechanical and durability properties.